Liquid crystal displays (LCDs) are used as display parts of computers, display parts of control panels of home appliances, and display parts of cellular phones and are required to further reduce power consumption, weight, and thickness.
The liquid crystal display is not a self-luminous device, and therefore, must have an external light source or environmental external light. The typical external light source is a backlight, which is a planar light source placed behind an LCD panel. The backlight is required to emit the outgoing light in the direction of an observer.
A typical configuration of a LCD panel with the side-incidence-type backlight is shown in FIG. 1. A light source 90 emits light, which is made incident on the side edge of a light guide 12, which obliquely emits light 14. The light 14 is bent with an optical sheet, typically a prism sheet 91 to the direction normal to the sheet plane and is diffused with a diffuser 32 so as to reduce chromatic dispersion and irradiate a liquid crystal panel 30 that displays an image. The shape of the light guide and the shape of the prism sheet 91 arranged between the light guide and the liquid crystal panel are optimized to increase the brightness in the normal direction.
In the vicinity of the light-entering part of a side-incidence backlight, the uniformity of the brightness is so inferior as to deteriorate the quality of the image displayed on the LCD panel placed over the backlight. Due to the low uniformity, the region within a certain distance from the light-entering part must be reserved as an undisplayed area, which hinders the miniaturization of LCD's. It is difficult for conventional prism sheets to simultaneously realize high brightness and high brightness uniformity. For example, by adding a diffusing structure to a prism sheet, the brightness uniformity can be improved, while the brightness is spoiled.
An output angle θo of light from the light guide is dependent on the design of the light guide. An incidence angle θi is usually 20° to 70°. The role of the prism sheet 91 is to efficiently bend the light to a direction where θo is 0°, i.e., the normal direction. For this, it is necessary to reduce Fresnel reflection that is interfacial reflection between an air layer and the prism sheet and make as much light as possible advance in the direction of 0°. As the emission from usual light guides has some angular distribution, it is favorable that the emission angle θo of an optical sheet changes smaller than the incident angle θi. Such an optical sheet can maintain higher brightness in the normal direction than the one with a fixed light-bending angle. Light from the light source is white light, and therefore, it is necessary to reduce the wavelength dependence of bend angle and minimize dispersion. The dispersion deteriorates the color reproduction of liquid crystals to degrade display quality.
The conventional prism sheet bends light by utilizing the characteristics of macroscopic optical parts, which function according to geometric-optical laws, such as refraction and total-internal reflection. With the macroscopic geometry, geometric-optical sheets, including the prism sheet, cannot help remaining thick. Thus, the prism sheet is an obstacle in the attempt to make thinner backlight. As each prism in the prism sheet independently functions to bend light, and defect or foreign matter in the prism results in an anomalous scattering of the light that passes through the prism. Anomalous stray rays can easily be recognized as some flaw in the display, for example, a bright spot. A display is sensitive to the defect or foreign matter and causes a display abnormity, to deteriorate product quality. To avoid the prism defect or foreign matter, the conventional prism sheet needs careful handling and manufacturing.
Compared with an element using the geometrical optical effect, an optical element (hologram optical element) using a diffraction/interference phenomenon based on wave optics has advantages that it can be thinned and that a plurality of functions such as condensing and diffusing can be realized with a single element. The hologram-optical elements known to date involve dispersion and high-order diffraction, and therefore, has been used not for bending white light but for diffusing white light to expand a viewing angle (refer to Japanese Patent Application Laid-Open Publication No. 7-114015 (page 1-2, representative drawing), Japanese Patent Application Laid-Open Publication No. 9-325218 (page 1-2, representative drawing), Japanese Patent Application Laid-Open Publication No. 10-506500 (page 1-4, FIG. 1-5), Japanese Patent Application Laid-Open Publication No. 11-296054 (page 1-2, FIG. 2-5), and Japanese Patent Application Laid-Open Publication No. 2000-39515 (page 1-2, FIG. 1-2)) and for separating white light (Japanese Patent Application Laid-Open Publication No. 9-113730 (page 1-5, representative drawing) and Japanese Patent Application Laid-Open Publication No. 10-301110 (page 1-2, FIG. 68). The effect of diffusing white light has been used for making dot-matrix display defects invisible (refer to Japanese Patent Application Laid-Open Publication No. 5-307174 (page 1-2, representative drawing), Japanese Patent Application Laid-Open Publication No. 6-59257 (page 1-2, representative drawing), Japanese Patent Application Laid-Open Publication No. 6-294955 (page 1-2, representative drawing), Japanese Patent Application Laid-Open Publication No. 7-28047 (page 1-2, representative drawing), and Japanese Patent Application Laid-Open Publication No. 7-49490 (page 1-2, representative drawing)). A method of designing a hologram optical element is described in, for example, Victor Soifer, Victor Kotlyar, and Leonid Doskolovich, “Iterative Methods for Diffractive Optical Elements Computation,” U.S.A., Taylor & Francis, 1997, p. 1-10.
Such a hologram optical element using the diffraction/interference phenomenon based on wave optics has problems of 1) producing diffracted light of orders of diffraction other than that at which incident light is vertically diffracted, 2) lowering the diffraction efficiency of the required order of diffraction, and 3) causing large wavelength dispersion. For example, if a period is small, there will be no order of diffraction for vertical diffraction or wavelength dispersion will be large. If a depth is improper, the diffraction efficiency of the required order of diffraction will be low.